1. Field of the Invention
The invention relates to coated abrasives formation. The invention particularly relates to materials and methods for the formation of coated abrasive materials and the curing of backing coats on cloth or paper which will be used for formation of coated abrasives.
2. Prior Practices
It is known in the coated abrasive art to apply resinous binder and abrasive grains to a paper or cloth substrate which is cured yielding sufficient strength for the following applications, subsequently the size coat is applied and the product completely cured. Suitable binders are for example glutelin glue, phenolic resins and, if water-proof papers are desired, polyurethane resins, epoxy resins and alkyd resins, possibly in combination with melamine resins. Special requirements as related to technique, apparatus and time are necessary for the curing process. To avoid destruction of the substrates usually consisting of polyester or cellulose, curing should be effected at a maximum temperature of 120.degree. to 130.degree. C. Rapid curing allowing for the use of a horizontal dryer is difficult, because of the formation of gas bubbles affecting the adhesion of the resin on the substrate. Drying of the coated material generally requires one or several hours, and is therefore carried out in a loop dryer. The loop dryers through which the coated web material is passing, enable a long drying process, but there are also disadvantages, such as the formation of defects where the material is suspended, sagging of the binder and changing of the grain position due to the vertical suspension, variation of temperature and the resulting inconsistent crosslinking of the binder produced by the necessary slow air circulation.
There are several disadvantages of the predominant commercial practice of forming coated abrasives. There are several curing steps in the typical process for formation of cloth-backed abrasives. The major areas of production may be considered as first the cloth treatment to prepare the base cloth for application of abrasives and second the making of the coated abrasives using the previously prepared base cloth. The base cloth is coated with at least one backing coat of resin which impregnates the cloth with resin and fills interstices in the back of the cloth. The backing cloth is also coated with at least one face coat that fills interstices of the cloth on the side where abrasive grain is placed. The face coat or coats of the backing cloth also aid in adhesion of the coat containing the grains onto the cloth.
The second major area of coated abrasive drying is the drying of the make coat which contains the grain and curing the size coat which is an overcoat placed onto the coated abrasive after the grain is at least partially adhered onto the backing by the make coat. In some instances, pre-size coat(s) are applied prior to the make coat. The drying of the presize make and size coats as set forth above generally is done in a lengthy loop dryer that requires a tremendous amount of floor space and energy. Further the larger dryers where the curing takes place over a long period are difficult to completely control for accurate temperature. There also is the problem of the resin and grain shifting positions during curing because of the long hang times in the uncured form.
It has been suggested in the U.S. Pat. No. 4,047,903, Hesse et al, that the formation of coated abrasives be carried out with at least one layer of resin being cured by electron beam. However, there has remained a need for apparatus which would allow the commercial exploitation of electron beam curing. Hesse et al does not set forth apparatus that would allow the continuous formation of coated abrasives. There are extensive difficulties in commercial exploitation of electron beam curing. Selection of resins suitable for electron beam curing of all resin coats utilized in formation of coated abrasives is difficult.
When forming coated abrasives, normally relatively thick coats of resin compared with prior uses of the electron beam are utilized. The resins necessary also are very sticky prior to being completely cured. Therefore, multiple path systems such as disclosed in some prior electron beam curing systems such as U.S. Pat. No. 3,022,543 are not satisfactory since if the resins touch a roller the system usually will gum-up and not perform.
Therefore, there remains a need for a system of electron beam curing resins which will allow formation of good quality coated abrasives in a low-cost commercially satisfactory manner.